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   FOUS11 KWBC 040819   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   319 AM EST Sun Jan 4 2026   
      
   Valid 12Z Sun Jan 04 2026 - 12Z Wed Jan 07 2026   
      
      
   ...Northern Plains/Upper Midwest/Great Lakes/Interior Northeast...   
   Days 1-3...   
      
   Some modest lake effect snow (LES) will continue through Sunday   
   aftn, but primarily within a single band south/southeast of Lake   
   Ontario with upstream connection to Lakes Huron and Superior.   
   Additional snowfall across this region should be light as reflected   
   by WPC probabilities for an additional 2+" of just 10-30%,   
   although brief 1"/hr rates are possible (10-30% chance).   
      
   Thereafter, the mid-level pattern flattens a bit as shortwave   
   ridging expands from the west, leading to pinched and more   
   progressive flow across the region. Within this flow, two back-to-   
   back shortwaves will traverse the region bringing rounds of wintry   
   weather.   
      
   The first of these will emerge from the High Plains Sunday morning   
   and push due east across the Great Lakes in a de-amplifying manner   
   as it becomes absorbed into the pinched westerlies. Despite the   
   weakening amplitude, height falls, PVA, and the left-exit region of   
   a strengthening jet streak will produce deep layer ascent, and   
   the intensification of this jet streak will likely result in   
   enhanced omega despite the weakening amplitude of the shortwave. At   
   the same time, increasing and impressive 925-850mb WAA will spread   
   northeast ahead of this impulse, aligning with the best left-exit   
   dynamics and the associated response to produce impressive fgen but   
   with limited temporal duration. Since this feature will be of   
   Pacific origin, the accompanying moisture will be impressive as   
   reflected by NAEFS PWs that are above the 97th percentile, aligned   
   with the strongest ascent. This suggests that as precipitation   
   expands from MN through the U.P., and into the northern L.P., a   
   narrow band of heavy snow will develop which has a 60-80% chance   
   of producing 1"/hr snowfall rates. The duration of these will,   
   again, be limited to the progressive nature of the wave, but WPC   
   probabilities for 4+ inches are above 50% from the Arrowhead of MN   
   eastward through the northern L.P. of MI, with locally 6-8"   
   possible (30% chance) in portions of the eastern U.P.   
      
   Additionally, south of this axis of heavy snow, a corridor of   
   modest mixed precipitation, including freezing rain, is likely as   
   the WAA surges a warm nose to around +3C at 850-800mb. The   
   refreezing layer is quite deep, so this may manifest more as sleet   
   than freezing rain, but hazardous travel is Sunday evening from MN   
   through central WI where WPC probabilities for 0.01" of ice are   
   50-70%.   
      
   This lead wave will continue to track into New England before   
   exiting to the Atlantic Monday night, but additional snowfall is   
   expected to be light, generally 1-3", although locally higher in   
   the Tug Hill Plateau through lake enhancement, as ascent weakens.   
      
   The next shortwave follows quickly in the wake of the first, and   
   emerges from CA Sunday evening before racing to reach the Corn Belt   
   by Tuesday morning. This impulse will track along the southern   
   subtropical jet, which is progged to intensify over the Central   
   Plains, reaching towards 140 kts. The favorable overlap of the LFQ   
   and height falls downstream of the shortwave will likely lead to   
   surface low pressure development Monday night near Iowa, with this   
   low then tracking steadily northeast into Ontario by Tuesday   
   evening. While there is still some spatial spread in the placement   
   of this low according to the different ensemble clusters, the   
   increasing ascent will lead to another round of wintry   
   precipitation from Minnesota through the Great Lakes (where a mix   
   of freezing rain and snow is likely), with additional mixed   
   precipitation spreading into New England by the end of the forecast   
   period. Total ascent with this secondary wave should be less   
   intense than the lead wave, and the thermal evolution is quite   
   complex due to potential secondary low development in the Gulf of   
   Maine, but at this time WPC probabilities for 2+ inches of snow are   
   just 10-30% in the higher elevations of Upstate NY and northern New   
   England.   
      
   For the freezing rain, WPC probabilities indicate a high risk   
   (>70% chance) of at least 0.01" of ice from central MN near the   
   Twin Cities through much of WI and MI, and into central and   
   northern New England. The greatest risk for at least 0.1" of ice is   
   likely in parts of northern WI, northern MI, and the higher   
   elevations of the Catskills, Adirondacks, and southern Greens.   
      
      
   ...Western U.S. Mountain Ranges...   
   Days 1-2...   
      
   The active west continues into early next week as an anomalous   
   trough persists just off the Pacific coast through Tuesday morning   
   before amplifying into a closed low which then undercuts towards   
   Baja by the end of the forecast period.   
      
   Downstream of this trough, nearly continuous S/SW flow will shed   
   spokes of vorticity onshore, with the accompanying height falls/PVA   
   leading to rounds of precipitation across much of the western   
   CONUS. At the same time, a subtropical jet will amplify and waver   
   from CA through the Central Plains, providing additionally enhanced   
   ascent. The combination of persistent SW mid-level flow with the   
   wavering jet stream will surge periods of elevated IVT onshore,   
   resulting in PW plumes that occasionally exceed the 90th   
   climatological percentile. While there may be periods of enhanced   
   ascent locally across different regions, in general the next 48   
   hours appear to be synoptically forced with broad large-scale   
   ascent, locally enhanced through PVA where the vorticity lobes   
   stream overhead, and through upslope enhancement. The strongest   
   upslope flow continues to be appear focused over the Sierra where   
   snowfall will be prolific, but heavy snow is likely through Monday   
   night across much of the region.   
      
   With broad SW flow encompassing the region, the accompanying WAA   
   will surge snow levels to as high as 6000-8000 ft in the Great   
   Basin and Rockies D1 (but generally 3000-5000 ft elsewhere), before   
   steadily falling through D2 as a cold front pushes eastward and the   
   trough axis finally swings farther east to the coast. By 12Z   
   Tuesday, snow levels are expected to be generally 1500-2500 ft in   
   the north, to 5000-6000 ft in the south. 2-day WPC probabilities   
   (12Z Sunday through 12Z Tuesday) feature a high risk (>70% chance)   
   for 12+ inches in the Sierra, northern CA ranges, Uintas, Tetons,   
   Wind Rivers, and portions of the Salmon River/Sawtooth/Bitterroots,   
   with lesser snowfall expected at the other regional terrain. In   
   the Sierra, 2-4 feet is possible in the higher elevations.   
      
      
   ...Cascades...   
   Day 3..   
      
   Mid-level flow across the Northern Pacific will become pinched and   
   zonal Tuesday, while a weak impulses approaches the coast and may   
   move onshore overnight into Wednesday morning. This zonal mid-level   
   flow will be topped by a strengthening Pacific jet streak which   
   will drop gradually southward such that the overlap of PVA/height   
   falls will match the greatest LFQ diffluence to provide ample   
   ascent. This will be aided by upslope enhancement into the terrain   
   thanks to the westerly flow, producing brief but impressive deep   
   layer lift into IVT that has a >80% chance of exceeding 250 kg/m/s.   
   While westerly IVT does not usually result in as impressive of   
   precipitation as that with a S or SW component, a fully saturated   
   column with steepening lapse rates will result leading to a period   
   of heavy snow in the Olympics and Cascades of OR and WA. Snow   
   levels will be generally 2000-3000 ft during this time, so pass   
   level impacts leading to hazardous travel are expected. WPC   
   probabilities for more than 8 inches of snow reach 70+%, with more   
   than 1 foot possible at the passes (Snoqualmie and Stevens   
   included).   
      
      
      
   Weiss   
      
      
   $$   
      
   = = =   
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